An organic electroluminescent (EL) display is a flat panel display for use as a computer or television monitor. A preferred method of driving an organic EL display is by an active matrix driving method, which provides a high-quality display image, while eliminating crosstalk. In an active matrix driving method, a thin-film transistor (TFT) is generally used as a switching element for an organic light emitting diode (OLED). OLED's are EL elements which operate by organic electroluminescence (EL).
FIG. 7 is a view showing a general constitution of an OLED pixel circuit driven by a TFT. With reference to FIG. 7, a conventional OLED pixel circuit includes an OLED 711 which is a light-emitting element, a driving TFT 712 for driving the OLED 711, a switching TFT 713 and a capacitor 714. A gate electrode of the driving TFT 712 is connected to the output of a switching TFT 713 and the capacitor 714. When a sufficient voltage is applied to the gate electrode of driving TFT 712, a driving electric current on a supply line 721 is supplied to the OLED 711 to allow the OLED 711 to emit light.
A gate electrode of the switching TFT 713 is connected to scan line 722. In accordance with a driving voltage on this scan line 722, a voltage obtained from a signal line 723 is applied to the gate electrode of the driving TFT 712. The capacitor 714 is connected to an output of the switching TFT 713 at one terminal, and also connected to a capacitor line 724 at another of its terminals. The capacitor 714 is charged by the switching TFT 713 and retains a voltage to be applied to the gate electrode of the driving TFT 712. Depending on the circuit arrangement, the capacitor line 724 may be arranged as a ground line, or the supply line 721 may be also used as the capacitor line 724.
TFT's have parasitic capacitance, attributable to their stacked structure, which includes electrodes, an insulating layer, a semiconductor layer and the like. In the switching TFT 713, a signal waveform (a scanning pulse) of the scan line 722 changes the electric potential retained by the capacitor 714 due to parasitic capacitance (Cgs) between a gate and a source of switching TFT 713. Such voltage which changes the electric potential of the capacitor 714 is referred to as a kickback voltage.
A change in voltage at the capacitor 714 is identical to the gate potential of the driving TFT 712 which drives the OLED 711. Accordingly, if the electric potential of the capacitor 714 declines, the driving electric current to be supplied to the OLED 711 is reduced, whereby emission luminance of the OLED 711 will be reduced.
FIG. 8 is a view showing the relationship between the signal waveform on the scan line 722, the electric potential of the capacitor 714, and the emission luminance of the OLED 711. As shown in the drawing, the signal waveform on the scan line 722 includes an addressing period when the switching TFT 713 is turned on, and a driving period when the switching TFT 713 is turned off. As shown in FIG. 8, when the scan line signal 722 changes from the addressing period to the driving period, the electric potential of the capacitor 714 is reduced in the amount of the kickback voltage, which in turn reduces the emission luminance of the OLED 711. Thus, in EL displays operated by an active matrix driving method using TFTs, the emission luminance of the OLED is reduced by the kickback voltage generated at each OLED pixel circuit arising from the parasitic capacitance of the switching TFT. In addition, the gate voltage of the driving TFT 712 changes due to the kickback voltage, and is amplified by the driving TFT 712 as a change in the driving electric current of the OLED.
The light emission characteristic of the OLED is very steeply dependent on the driving voltage. For this reason, the decrease in the gate voltage of the driving TFT attributable to the kickback voltage greatly decreases the emission luminance of the OLED, whereby correct gradation display is impeded. Moreover, display unevenness occurs over the entire organic EL display.
FIG. 9 is a graph illustrating the relationship of the driving voltage to the light emission characteristic of the OLED. Referring to FIG. 9, a small change in the driving voltage (delta)Vkb, in an amount comparable to the kickback voltage, effects a large change in the emission luminance of the OLED.
In order to reduce the effect of kickback voltage on the gate voltage of the driving TFT, one might increase capacitance by increasing the size of capacitor 714, such that change due to the kickback voltage is reduced. However, since capacitor 714 is formed on the scan line in an actual OLED pixel circuit, it is necessary to increase a width of the scan line in order to increase capacitance. That is undesirable, as it leads to a decrease in the emission-contributable area of the OLED pixel circuit instead.
Another way might be to increase the electric current supplied to the OLED to cope with reduced emission efficiency due to decrease in the emission-contributable area. However, the OLED (the organic EL) deteriorates faster under increased current density, thus shortening its lifetime. Therefore, increasing the electric current is not desirable.